1. Field of the Invention
This invention relates to a radiation image read-out method and apparatus, and a stimulable phosphor sheet for use in the radiation image read-out method and apparatus. This invention particularly relates to a radiation image read-out method and apparatus, wherein two image signals are detected from the opposite surfaces of a stimulable phosphor sheet, and a stimulable phosphor sheet for use in the radiation image read-out method.
2. Description of the Prior Art
It has been proposed to use stimulable phosphors in radiation image recording and read-out methods. Specifically, a radiation image of an object, such as a human body, is recorded on a stimulable phosphor sheet, which comprises a substrate and a layer of a stimulable phosphor overlaid on the substrate. The stimulable phosphor sheet, on which the radiation image has been stored, is then scanned with stimulating rays, such as a laser beam, which cause it to emit light in proportion to the amount of energy stored thereon during its exposure to the radiation. The light emitted by the stimulable phosphor sheet, upon stimulation thereof, is photoelectrically detected and converted into an electric image signal. After the image signal has been detected from the stimulable phosphor sheet, the stimulable phosphor sheet is exposed to erasing light or heat, and energy remaining on the stimulable phosphor sheet is thereby released.
The image signal, which has been obtained with a radiation image read-out method, is then subjected to image processing, such as gradation processing or frequency processing, such that a visible radiation image having good image quality can be reproduced and used as an effective tool in, particularly, the accurate and efficient diagnosis of an illness. The image signal, which has been obtained from the image processing, is then reproduced as a visible image on photographic film or on a cathode ray tube (CRT) display device and used in making a diagnosis, or the like. The stimulable phosphor sheet, from which the image signal has been detected, is exposed to erasing light or heat, and energy remaining on the stimulable phosphor sheet is thereby released. The erased stimulable phosphor sheet can then be used again for the recording of a radiation image.
Also, techniques for detecting light emitted by the opposite surfaces of a stimulable phosphor sheet have been proposed in, for example, U.S. Pat. No. 4,346,295 and Japanese Unexamined Patent Publication No. 4(1992)-280060. With the proposed techniques, the substrate of the stimulable phosphor sheet is constituted of a transparent material capable of transmitting light, which is emitted by the stimulable phosphor layer of the stimulable phosphor sheet, and the light emitted by the stimulable phosphor layer is detected approximately simultaneously from the opposite surfaces of the stimulable phosphor sheet. In this manner, the efficiency, with which the light emitted by the stimulable phosphor sheet is collected, is kept high. Further, the image signal components of two image signals having been detected from the opposite surfaces of the stimulable phosphor sheet are added together in a predetermined addition ratio, which image signal components represent corresponding picture elements on the front and back surfaces of the stimulable phosphor sheet. In this manner, the signal-to-noise ratio is kept high.
As described above, in the systems for detecting light emitted by the opposite surfaces of a stimulable phosphor sheet and adding the two obtained image signals to each other, the image signal components of the two image signals having been detected from the opposite surfaces of the stimulable phosphor sheet are added to each other, which image signal components represent corresponding picture elements on the front and back surfaces of the stimulable phosphor sheet. The systems for detecting light emitted by the opposite surfaces of a stimulable phosphor sheet and adding the two obtained image signals to each other are set such that the addition may be carried out automatically and regardless of whether each of the two image signals is or is not an appropriate image signal, which has been detected accurately. Therefore, for example, in cases where a failure has occurred with a photomultiplier of an image information read-out system or with a stimulating ray irradiating system, which is located on one surface side of the stimulable phosphor sheet, the detection of the image signal from the corresponding surface of the stimulable phosphor sheet cannot be carried out accurately. Accordingly, an image signal appropriate for use in the reproduction of a visible image cannot be obtained from the corresponding surface of the stimulable phosphor sheet. However, in the aforesaid systems for detecting light emitted by the opposite surfaces of a stimulable phosphor sheet and adding the two obtained image signals to each other, even if the failure described above has occurred, the image signal components of the inaccurate image signal, which has been detected from the one surface of the stimulable phosphor sheet, and the accurate image signal, which has been detected from the other surface of the stimulable phosphor sheet, are automatically added to each other, which image signal components represent corresponding picture elements on the front and back surfaces of the stimulable phosphor sheet. An addition image signal, which has been obtained from the addition process, is then used for reproducing a visible image, which is to be used in making a diagnosis, or the like.
However, the visible image for use in making a diagnosis, or the like, which is reproduced from the image signal obtained on the surface side of the stimulable phosphor sheet, on which an abnormal state in the image readout has occurred, is inaccurate and inappropriate for use in making a diagnosis, or the like. Also, if such an inaccurate visible image is reproduced, an accurate diagnosis, or the like, cannot be made. Further, the operation for recording a radiation image must be carried out again. In such cases, considerable time and labor will be required.
Also, in cases where an abnormal state in the image readout has occurred as described above, it is useless to carry out the operation for detecting the image signal in the abnormal state.